1. Field of the Invention
The present invention relates to a multi-color image display such as a display monitor or a color television, and more particularly, to a method for compensating for the luminance of a color signal, which is performed by a multi-color image display, and an apparatus for compensating for the luminance of a color signal, which is included in the multi-color image display.
2. Description of Related Art
In a multi-color image display which displays four of more primary colors, synthetic colors created by mixing three or more primary colors, for example, achromatic color series are high in luminance, while color signals of primary color series are relatively low in luminance as compared to the synthetic colors. Here, the color signal of primary color series means each primary color or color signals near to a line or an area connecting two primary colors. Especially, in a case where the number of the primary colors is more than four or four, a difference between the luminance of a color signal of primary color series and that of a synthetic color becomes greater.
FIG. 1 shows an example in which a color gamut of a multi-color image display representing four primary colors is illustrated on a two-dimensional plane using a red-color (R) axis and a green-color (G) axis.
Referring to FIG. 1, an arbitrary color signal represented by the multi-color image display has a unique coordinate within a hexahedron Org-Q3-Q1-Q4-Q5-Q6 in a color coordinate space. Generally, a three-color image display is a hexahedron in a three-dimensional space and a square in a two-dimensional space, while a multi-color image display is a heptahedron or a polyhedron with more faces than the heptahedron in the three-dimensional space and is a pentahedron or a polyhedron with more faces than the pentahedron in the two-dimensional space. Also, color signals with low saturation are distributed in the interior of the color area Org-Q1-Q4-Q5 shown in FIG. 1 and vector lengths (or, luminance magnitude or luminance amount) of the color signals within this color area Org-Q1-Q4-Q5 have specified values. Meanwhile, color signals with high saturation are distributed in the interiors of color areas Org-Q3-Q1 and Org-Q6-Q5, and vector lengths of the color signals within the color areas have values smaller than the specified values and are changed according to the saturation. That is, if two color signals have the same luminance and different saturations and at least one of the two color signals is placed in the interior of the color area Org-Q3-Q1 or Org-Q6-Q5, the two color signals are displayed differently in luminance and saturation on a four-color image display. Accordingly, deterioration of picture quality may occur due to falling-off in the relative luminance of a color signal with high saturation.
FIG. 2 is a view in which the color gamut of the multi-color image display of FIG. 1 is illustrated on a two-dimensional plane with a saturation axis and a luminance ratio axis, wherein Q1 through Q6 correspond respectively to Q1 through Q6 shown in FIG. 1.
In a saturation range of Q1-Q4-Q5 shown in FIG. 2, a constant luminance ratio is maintained, but in a saturation range of Q1-Q3 or Q5-Q6, the smaller the luminance ratio is, the higher the saturation is. This is because as the number of primary colors used in an image display increases, a light-emitting time or light-emitting area of each primary color is shorter or smaller, which deteriorates the luminance of the primary color.
Hereinafter, conventional luminance enhancing methods will be described.
One among conventional luminance enhancing methods is disclosed in U.S. Pat. No. 4,717,953 entitled “Bightness Control Circuit for a Television Receiver”. This conventional luminance enhancing method increases the luminance of an image signal only by applying offset values to RGB. However, this conventional method brightens entire colors due to the use of offset values, thereby deteriorating the contrast of an image.
Another one among the conventional luminance enhancing methods is a method using a gamma function disclosed at Internet address http://www.inforamp.net/Npoynton/GammaFAQ.html. According to this conventional luminance enhancing method, a gamma function can be applied only to a luminance component Y as in a broadcast standard or can be applied to each of components of RGB. In a case of the former, a RGB signal is divided into a luminance signal Y and two color difference signals (Cb and Cr) or (I and Q), a gamma function is applied to the luminance signal Y to increase the luminance, and then the result is converted to an RGB signal. However, since this conventional luminance enhancing method performs color enhancing on entire color areas, a luminance difference between a color with high saturation and a color with low saturation exists.
Still another one among the conventional luminance enhancing methods is a method using a histogram equalization disclosed in the title of “Digital Image Processing” by W. K. Pratt, published in Wiley's publishing company, in 1978, and in the title of “Digital Image Processing” by R. C. Gonzalez and R. E. Woods, published in Addition-Wesley's publishing company located in Massachusetts, in 1993. The above-mentioned conventional luminance enhancing methods increase the luminance and contrast throughout all ranges of pixel values in an image. As such, since the conventional luminance enhancing methods perform luminance enhancing on entire color areas, a luminance difference between a color with high saturation and a color with low saturation still exists.
As described above, since a luminance difference according to saturations still exists in the conventional luminance enhancing methods, in particular, a color with higher saturation is smaller in the amount of luminance increase compared to a color with lower saturation, deterioration of picture quality may occur when colors are reproduced in an image display.